The present invention relates generally to coding systems for digital communications, and particularly to pruning methods for the obtention of S-random interleavers with a reduced size starting from an initial S-random interleaver having a larger size.
In the present description, following an already established practice in this field, by “interleaver” it is meant the “interleaver permutation” or “interleaver law” associated with an interleaver device in the proper sense of the word.
Interleavers play a crucial role in systems using turbo-codes.
Interleavers of the S-random type represent, as it is well-known, an optimum class of interleavers, and differently from many other permutation systems, are sufficiently robust with respect to the specific convolutional codes employed and to the puncturing rate applied to the overall code.
Many application systems require a great flexibility in terms of block length and code-rate, and the change of these parameters involves a corresponding modification of the interleaver size. In such cases, it is highly recommendable to obtain, by use of an algorithm, all the needed interleavers from a mother interleaver which exhibits the largest size, avoiding the need to store all the necessary permutation laws.
Unfortunately the known pruning techniques disclosed in the literature generally destroy the properties of S-random interleavers.
A pruning method is disclosed in EP 1 257 064 A and in M. Ferrari, F. Scalise, S. Bellini, “Prunable S-random Interleavers”, in Proc. IEEE Conf. Communications, Vol. 3, 2002, pages 1711-1715.
The pruning method disclosed in said documents provides for discarding all the elements of an initial interleaver which have a value greater than the size of the desired smaller interleaver. That technique allows to store one single interleaver and for the larger interleavers it affords (only) the same spread properties of the smaller interleaver.